A conventional thermal fixing device for an electro-photographic type image forming device includes a tubular fusing film, a heater disposed in an internal space of the fusing film, a pressure roller, a nip plate providing a nip region between the pressure roller and the fusing film, and a stay surrounding the heater and supporting the nip plate. In such a fixing device, the heater heats the nip plate to heat a surface of a recording sheet in cooperation with the fusing film, so that a developing agent image formed on the recording sheet is thermally fixed.